Bikes been making this noise for quite a while. the only time it makes the noise is when its on a real low idle. if i barely touch the trottle the noise goes away. i checked the flywheel and it is fine. it kind of sounds like the noise is in the case directly behind the sprocket. bike has oil in it and no metal shavings in in the oil. the bike runs and shifts fine. any help would be appreciated. thanks.